P1006: CHARACTERIZATION OF TL-895: A NOVEL BRUTON TYROSINE KINASE INHIBITOR (BTKI) IN CLINICAL DEVELOPMENT FOR CHRONIC LYMPHOCYTIC LEUKEMIA (CLL) AND MYELOFIBROSIS (MF)
Authors: Michael Gulrajani, Tracy Clevenger, Jean Cheung, Fanny Krantz, Cynthia Nguyen, Miranda Luarca, Cecile Krejsa, Wayne P. Rothbaum, Todd Covey
Journal: HemaSphere (2023)

Topic: 15. Myeloproliferative neoplasms - Biology & Translational Research Background: BTK, a key component of B-cell receptor (BCR) signaling, promotes the interaction of CLL cells with the tumor microenvironment contributing to disease-related lymphadenopathy, splenomegaly, hepatic infiltration, and elevated lymphocyte counts (Montresor 2018). In MF, activated JAK2 increases BTK and NFκB signaling leading to aberrant CD34+ cell trafficking and dysregulated cytokines (Nimmagadda 2019; Fisher 2019). Novel and improved BTK inhibitors may induce deeper and more durable responses in CLL and offer the potential to improve clinical outcomes in MF. TL-895 is a highly potent, selective, orally available, covalent, small molecule inhibitor of BTK and bone marrow tyrosine kinase X-linked (BMX). TL-895 is being studied in patients (pts) with CLL (NCT02825836) and is the first BTKi being investigated in pts with MF (NCT04655118). Aims: To profile the activities of TL-895 in myeloid, B and T cells in functional assays, and to compare the pharmacodynamic (PD) impact in CLL and MF pts. Methods: TL-895 activity was assessed with respect to: (i) potency across BTK-related kinases, (ii) BTK levels and protein turnover in lymphoid and myeloid cells, (iii) signaling in CLL and MF cells (Ramos, Hel-92) and healthy peripheral blood mononuclear cells (PBMC), (iv) cytokine production in GM-CSF treated monocytes, (v) chemotaxis of MF cells toward SDF-1, and (vi) Ca2+ flux and IFNɣ production in stimulated T cells. PD analyses of BTK occupancy, pBTKY223 inhibition, cell signaling/activation, and cytokine levels were performed in CLL and MF pt samples. Results: TL-895 showed high selectivity with limited off-target activity against structurally-related kinases with high potency inhibition of BTK (4.9nM) and BMX (1.6nM), moderate inhibition of BLK, ERBB4, TEC and TXK (10-39nM), and no inhibition of CSK and EGFR. BTK protein was highly expressed in lymphoid and myeloid cell lines, but protein turnover was notably faster in myeloid cells (89% vs 26%/day). As expected, TL-895 significantly inhibited pBTK in healthy B and Ramos cells (Fig 1A) and potently inhibited pBTK in healthy monocytes and Hel-92 cells (Fig 1B). TL-895's impact on cell function was pronounced. CD69, an activation marker, was downregulated in B cells from BCR-stimulated PBMC and whole blood (12nM and 21nM EC50, respectively), while IL-8, IL-1β, MCP-1, MIP-1α, and IL-6 production was inhibited in healthy monocytes (1-3nM EC50). Interestingly, TL-895 also reduced chemotaxis in Hel-92 cells, preventing migration toward SDF-1 (p<0.05). T cell activation was not impaired at clinically relevant levels (≤1µM). In CLL and MF pts treated with TL-895 at 150 mg BID, BTK occupancy at trough was 97% and 95% respectively (Fig 2A, 2B), with corresponding reductions in pBTK in B-CLL and MF cells. Proinflammatory cytokines commonly elevated in CLL and MF, MIP-1β and IL-8, decreased substantially during the first treatment cycle (Fig 2C, 2D). Summary/Conclusion: TL-895, a highly selective, novel BTKi demonstrated potent inhibition of cell activation, proinflammatory signaling, migration and cytokine production in lymphoid and myeloid cells. In pts with CLL and MF, the 150mg BID dose achieved complete and sustained BTK occupancy throughout the dosing interval, despite faster BTK resynthesis in myeloid cells. TL-895 holds therapeutic promise by modulating key signaling nodes of cell activation, reducing stromal support, and downregulating proinflammatory cytokines in pts with CLL and MF.Keywords: Cytokine, Myelofibrosis, Chronic lymphocytic leukemia